Conventionally, compounds having a crystal structure called a “garnet structure” have been known (e.g., see Non-Patent Document 1).
The compound represented by the chemical formula Y3Al3O12 is one of such compounds. Y3Al5O12 is widely known as YAG, an abbreviation for yttrium aluminum garnet, and has been used in solid-state lasers, translucent ceramics, phosphors, and the like. Further, it is also known that there are compounds obtained by replacing the lattice position of Y of YAG with other metal element, particularly with a rare earth element, and compounds obtained by replacing the lattice position of Al of YAG with other metal element, particularly with Ga (e.g., see Non-Patent Document 2).
Herein, inorganic phosphor materials as compounds based on the compound represented by the chemical formula Y3Al3O12 and to which ions serving as luminescent centers, for example, rare earth ions and transition metal ions typified by Ce3+, Tb3+, Eu3+, Mn2+, Mn4+, Fe3+, and Cr3+, are added are defined as YAG-type phosphors (i.e., phosphors belonging to the yttrium aluminum garnet type), inorganic phosphor materials as compounds based on those obtained by replacing Y of YAG with La and to which ions serving as luminescent centers are added are defined as LaAG-type phosphors (i.e., phosphors belonging to the lanthanum aluminum garnet type), and inorganic phosphor materials as compounds based on those obtained by replacing Y of YAG with Lu and to which ions serving as luminescent centers are added are defined as LuAG-type phosphors (i.e., phosphors belonging to the lutetium aluminum garnet type). Further, these inorganic phosphor materials are defined all together as rare earth aluminum garnet type phosphors.
Further, for the sake of convenience, YAG-type phosphors activated at least with Ce3+ are defined as YAG:Ce-type phosphors, LaAG-type phosphors activated at least with Ce3+ are defined as LaAG:Ce-type phosphors, and LuAG-type phosphors activated at least with Ce3+ are defined as LuAG:Ce-type phosphors. Further, these phosphors are defined all together as rare earth aluminum garnet type Ce phosphors.
That is, the term YAG-type phosphor as used herein refers to a phosphor obtained by adding an ion serving as a luminescent center to an inorganic compound having a garnet type crystal structure and including at least yttrium, aluminum and oxygen as elements constituting the crystal lattice.
Examples of such phosphors include the compounds represented by the following chemical formulas.
(Y,Ce)3Al5O12 (e.g., see Patent Documents 1 and 2)
(Y,Gd,Ce)3Al5O12 (e.g., see Patent Documents 1 and 2)
(Y,Eu)3Al5O12 (e.g., see Patent Documents 1 and 2)
(Y,Tb)3(Al,Ga)5O12 (e.g., see Non-Patent Document 2)
(Y,Tb)3Al5O12 (e.g., see Non-Patent Document 2)
(Y,Ce,Pr)3Al5O12 (e.g., see Patent Document 3)
(Y,Lu,Ce,Pr)3Al5O12 (e.g., see Patent Document 4)
(Y,Ce)3(Al,Si)5(O,N)12 (e.g., see Patent Document 5)
(Y,Ba,Ce)3(Al,Si)5O12 (e.g., see Patent Document 6)
Further, LaAG-type phosphors as mentioned above are phosphors obtained by adding an ion serving as a luminescent center to an inorganic compound having a garnet type crystal structure and including at least lanthanum, aluminum, and oxygen as elements constituting the crystal lattice. Examples of LaAG-type phosphors include those mentioned above as examples of YAG-type phosphors, but Y in each of the chemical formulas is replaced with La.
Further, LuAG-type phosphors as mentioned above are phosphors obtained by adding an ion serving as a luminescent center to an inorganic compound having a garnet type crystal structure and including at least lutetium, aluminum, and oxygen as elements constituting the crystal lattice. Examples of LuAG-type phosphors include those mentioned above as examples of YAG-type phosphors, but Y in each of the chemical formulas is replaced with Lau.
Of these rare earth aluminum garnet type phosphors, YAG:Ce-type phosphors in particular are known to get excited when they are irradiated with corpuscular rays or electromagnetic waves such as electron beams, vacuum ultraviolet rays and blue light and emit yellow to green visible light. Further, it is also known that their 1/10 persistence time is 100 ns or less, meaning that they have ultra-short persistence characteristics. For these reasons, YAG:Ce-type phosphors have been used broadly in many light-emitting devices (e.g., see Non-Patent Document 2, and Patent Documents 1 to 7).
For example, a (Y,Ce)3Al5O12 phosphor (generally referred to as a Y3Al5O12:Ce3+ phosphor), whose composition is simple among the aforementioned YAG:Ce-type phosphors, emits yellow-green light. If any of the elements constituting Y3Al5O12Ce3+ is partially or entirely replaced with another element in the same group, for example, if Y is replaced with Lu or Gd, or Al is replaced with Ga, the color of light emitted by the phosphor changes as follows. That is, the color of light emitted by the phosphor changes to green if Al is replaced with Ga or Y is replaced with Lu, and to yellow to orange light if Y is replaced with Gd. Moreover, when an (AlO4) group constituting Y3Al5O12Ce3+ is partially replaced with a (SiO4) group, and Al is partially replaced with Mg to compensate for charge, the phosphor emits yellow to orange to red light (see Patent Document 8).
YAG:Ce-type phosphors, whose light colors have been controlled by partially replacing Y or Al with another element as described above, are used heavily in light-emitting devices used as light sources for display devices and for illumination devices, where primary light comes from solid-state light-emitting elements, such as light-emitting diodes (LEDs) and semiconductor laser diodes (LDs), and is emitted after being shifted toward the long wavelength side by the YAG:Ce-type phosphors (e.g., see Non-Patent Documents 2 and 3). In this way, in the realm of light-emitting devices that use green light or the like emitted by YAG-type phosphors, most notably by YAG:Ce-type phosphors, directly for illumination purposes or as a display light source, the development of new light-emitting devices is gaining momentum by partially or entirely replacing constituent elements of the crystal lattice of YAG:Ce-type phosphor with other elements with a different ion radius to control the wavelength (e.g., see Patent Documents 1, 2, and 11). For the purpose of obtaining yellow or orange emission, a composition including Gd in the crystal lattice or a composition including Mg and Si in the crystal lattice is used to control the hue.
In addition to YAG:Ce-type phosphors, several other phosphors that have a garnet structure are also known. Examples of such phosphors include a Ca3Sc2Si3O12:Ce3+ (CSS for short) green phosphor (see Patent Document 9), and a Tb3Al5O12:Ce3+ (TAG for short) yellow-green phosphor (see Patent Document 10). These CSS and TAG may be able to replace YAG:Ce-type phosphors, and are used in the field of white LEDs or their use in the field of white LEDs has been studied.